The Lumia Camera can no* longer be downloaded normally through the store. If you have downloaded it before, then check your Store apps under "My Apps" and try to re-download it.

Otherwise, the new default Microsoft Camera (just "Camera") works just as good as, if not better than, Lumia Camera. Specifically, launch time has improved, while preserving the resolution capture. New features have also been added under the settings.
Update: Looks like Lumia Camera can indeed still be downloaded from the store, thanks to pankaj981 below.